1. Field of the Invention
The present invention concerns, in general, the magnetic resonance tomography (MRT) as used in medicine to examine patients. The present invention concerns, in particular, an MRT imaging procedure for the acquisition of high-resolution single-shot images in a short scanning period.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear spin resonance and has been successfully employed in medicine and biophysics as an imaging procedure for more than 15 years. In this modality, the object is exposed to a strong, constant magnetic field. The nuclear spins of the atoms, which were previously randomly oriented, thereby are aligned in the object. Radio-frequency energy can then excite these “ordered” nuclear spins to a specific oscillation. This oscillation creates the actual measurement signal in MRT, which is recorded using suitable receiving coils. By the use of inhomogeneous magnetic fields, generated by gradient coils, the examination subject, and the signals therefrom, be encoded spatially in all three spatial directions, which is in general called “spatial encoding.”
The acquisition of the data in MRT takes places in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by means of Fourier transformation. The spatial encoding of the object, which spans k-space, takes place by means of gradients in all three spatial directions. A distinction is made between the slice readout gradient (determines an absorption slice in the object, usually along the z-axis), the frequency coding gradient (determines a direction in the slice, normally along the x-axis), and the phase-coding gradient (determines the second dimension within the slice, usually the y-axis).
First, a slice of the subject, e.g. in the z direction is selectively stimulated. The coding of the location information in the slice takes place by a combined phase and frequency coding by means of these two already-mentioned orthogonal gradient fields that in the example of a slice stimulated in the z-direction, are generated by the aforementioned gradient coils in the x and y-directions, respectively.
A first possible sequence for recording the data in an MRT scan is shown in FIGS. 2a and 2b. The sequence used is a spin-echo sequence. In this sequence, the magnetization of the spins in the x-y plane is displaced by a 90° excitation pulse, In the course of time (½ TE; TE is the echo time), this leads to a de-phasing of the magnetization components, which together form the transverse magnetization in the x-y plane Mxy. After a certain period of time, (e.g. ½ TE) a 180° pulse is emitted such that the de-phased magnetization components are flipped without the precession direction and precession speed of the individual magnetization portions being changed. After such a further time duration ½ TE, the magnetization components point in the same direction again, i.e. a regeneration or “re-phasing” of the transverse magnetization occurs. The complete regeneration of the transverse magnetization is called spin echo.
In order to measure an entire layer of the object to be examined, the imaging sequence is repeated N times for different values of the phase encoding gradient e.g. Gy, with the frequency of the magnetic resonance signal (spin-echo signal) being scanned, digitized, and stored N times in equidistant time intervals Δt in the presence of the readout gradient for each sequence execution using a Δt-clocked ADC (Analog Digital Converter). In this manner, a line-by-line numerical matrix (matrix in k-space or k-matrix) with N×N data points is obtained in accordance with FIG. 3b (a symmetrical matrix with N×N points is only one example, asymmetrical matrices also can be created). An MR image of the slice in question with a resolution of N×N pixels can be reconstructed directly from this data record through a Fourier transformation.
The scanning of the k matrix (or k matrices in the case of data acquisition from several layers) for spin echo sequences with diagnostically usable image quality normally requires several minutes of measurement time, was can be a problem for many clinical application. For example, patients cannot remain immobile for the required period of time. For examinations of the thorax or in the pelvic region movement of the anatomy is generally unavoidable (cardiac and respiratory movements, peristalsis). One way to accelerate the spin echo sequence was published in 1986 as the Turbo Spin Echo sequence (TSE sequence) or under the acronym RARE (Rapid Acquisition with Relaxation Enhancement) (J. Hennig et al. Magn. Reson. Med. 3, 823-833, 1986). In this procedure, that is much faster compared to the conventional aforementioned spin echo procedure, several multiple echoes are created based on a 90° excitation pulse, with each of these echoes being individually phase-encoded. A corresponding sequence diagram is shown in FIG. 4a for the case of seven echoes being generated for each. The phase-coding gradient must be switched before and after the echo according to the selected Fourier line. In this manner, a linear scanning of the k matrix takes place after one single RF excitation pulse (90°) as shown in FIG. 4b. The required total measurement time is shortened in this example by a factor of 7. The signal progression in FIG. 4a is shown in an idealized manner. In reality, the later echoes have increasingly smaller amplitudes due to the dismantling of the transverse magnetization T2.
An even faster imaging sequence is a combination of RARE with the half-Fourier technique that was introduced in 1994 as the so-called HASTE sequence (Half-Fourier Acquired Single-Shot Turbo Spin Echo) (B. Kiefer et al., J.Magn. Reson. Imaging, 4(P), 86, 1994). HASTE uses the same basic technique as RARE, but only half of the k-matrix is scanned. The other half of the k-matrix is reconstructed by calculation manner using a half-Fourier algorithm. For this purpose, use is made of the fact that the data points of the k-matrix are arranged mirror-symmetrical to the mid-point of the k-matrix. Thus, it suffices to only measure the data points of one k-matrix half and to complete the raw data matrix by mirroring with respect to the mid-point (and complex conjugation). In this manner, the measurement time can be reduced by half. The reduction of the recording time, however, is has an associated degradation of the signal to noise ratio (S/R) by a factor of √42.
A further method for quickly obtaining and scanning the k-matrix is the “echo-planar imaging” (EPI) procedure. The main idea behind this procedure is to generate a series of echoes in the readout gradient (Gx), which are assigned to different lines in the k-matrix through a suitable gradient switch (modulation of the phase encoding gradient Gy), after a single (selective) RF stimulus in a very short period of time. In this manner, all lines of the k-matrix can be acquired with one single sequence execution. Different versions of the method also known as the echo-planar technique differ only as to how the phase-encoding gradients are switched, i.e. how the data points of the k-matrix are scanned.
The ideal form of an echo-planar pulse sequence is shown in FIG. 3a. The needle-like Gy pulses in the activation of the readout gradient Gx (so-called “blips”) lead to the serpentine-like path of the k-matrix shown in FIG. 3b so that, with chronologically similar scanning, the measurement points come to lie equidistantly in k space.
The scanning of the echo sequence must be completed in a time that corresponds with the dismantling of the transverse magnetization T2*. Otherwise, the different lines of the k-matrix would namely be weighted according to the sequential order of their acquisition: certain location frequencies would be overemphasized and others underemphasized.
Another quick MRT imaging procedure is GRASE (Gradient and Spin-Echo) GRASE was first introduced in 1995 (D. Feinberg et al. Magnetic Resonance Medicine 33, 529-533, 1995) and can be seen as a hybrid technique of EPI and RARE with the underlying idea of scanning the spin-echo envelope with several gradient echoes. With GRASE, as shown in FIG. 5a, several RF 180° refocusing pulses are applied in order to generate a spin-echo string like with RARE as well as a number of readout gradient pulses in order to create the corresponding gradient pulse string after each respective 180° refocusing pulse. The diagram in FIG. 5b shows the chronological sequence in which the k-space lines are acquired during a GRASE sequence. The diagram shows the simplified case of 3 spin echoes (SE), 3 gradient echoes (GE) and thus a total of 9 phase-encoding steps. The chronological progression of the scanning takes place such that, for each spin echo, the corresponding gradient echoes and the k-matrix as thus filled up component by component. For clarification, the current scanning sequence is given in the right margin of the k-matrix. It is noted that other versions of GRASE that employ a different scanning sequence are also known.